OT: ok the maths one was too easy, try this

[The Natural Philosopher]

These amount to exactly the same thing.

The baloon and te air around it are initilly all in the same frame and in fact remain so, apart from the fact that the string moves forawad faster - due to the speed of sound in the truck body being MUCH faster than the speed of sound in the air inside it - assuming it has rear wheel drive.

Coinsedieran an alternative case, the trick is in fact a giant rubber condom full of water inside with a buoy is suspended, and ot has fromnt wheel drive.

Under accelerartion it simply leaves the rear wheels standing...

[The Natural Philosopher]

I am afraid that real life IS like the cartoon

Due to elasticity/speed of sound issues.(which turn out to be closely related)

[The Natural Philosopher]

If you can change the planet, you can change the transport, and who said it was a human?

"a Quargon is orbiting his 9000G high gravity methane atmosphered planet Quargin as bleaghship and an altitude of 47,000 zurds, how many miles away is the horizon?"

A. He cant, Quargons sense the world by direct grokking and are unsighted.

B. 3 feet because the stingship doesn't have portholes.

C. right round the planet because the refractive index variation of methane under a 9000G field and adiabatic temperature variations is enough to make the horizon above the line of sight. Any Quarg can see the whole planet firm anywhere except inside his house.

D. no answer, because the Quargons don't have a word for horizon.

[Andrew Gabriel]

"I start with a pint glass full of lemonade."

Students? Yea, right... Pull the other one.

[dennis]

Rubbish, I have stated what happens at the start and when its close to equilibrium if it ever can as its a very powerful engine so it may be able to accelerate until the speed of light, which I haven't commented on (yet).

He is answering the question as intended.

Odd its you that is being narrow minded and not including the starting conditions and being fixated on the "steady" state.

[John Rumm]

Actually that would depend on the speed of sound in air vs the speed of sound in string.

[John Rumm]

Constant acceleration would be equilibrium.

Funny how you get the wrong answer then

Well as you have see from all the demonstrations, the balloon does the opposite of what you predict.

If the question really was focussed on the starting point only, there would be no point in specifying a helium balloon with buoyancy in the first place - a lead weight dangling on a string would have sufficed and posed a somewhat less interesting question.

[dennis]

I've not seen a string made out of gas so its fairly safe to assume its quicker in the string. There would be the question of elasticity and breaking strain as the question states a very powerful engine it might just snap the string or burst the balloon then all sorts of things could happen.

[Richard Tobin]

Certainly in the steady state, i.e. once the truck's acceleration is constant.

The truck *starting* to accelerate will be like the direction of gravity changing. If that happens, it's not immediately obvious to me what the instantaneous effect on the ballon in the atmosphere is.

The change in the direction of gravity on the balloon would be instantaneous (well, speed of light), but the effect of buoyancy would depend on the speed of the air molecules, so it seems there should be a very short period when it would move the other way.

In fact, I'm now convinced it would initially move backwards, because in a vacuum it would move backwards, and the relative densities cannot have any effect until an air molecule hits it. This would be a very short time though!

-- Richard

[Tim Streater]

There's no reason at all why there shouldn't be a step function in the truck's acceleration.